For inputting/outputting optical signals to/from an optical circuit comprising optical waveguides on a substrate, it may be desired to provide the optical circuit with a function for outputting light propagated through an optical waveguide to the outside of the optical waveguide, and/or a function for inputting external light into an optical waveguide. A grating coupler is an optical input/output component that can be used for the above purposes; and it can input light propagated through an optical waveguide on a substrate to an end surface of an optical fiber positioned adjacent to a surface of the substrate, or it can input, in a reverse manner, light from an optical fiber to an optical waveguide.
FIG. 1 is a schematic cross-section view of a grating part in an example of a grating coupler having a construction for outputting guided light to the outside of an optical waveguide. In the following description, the construction, functions, and so on of the grating coupler will be briefly explained with reference to FIG. 1.
FIG. 1 is a schematic view of an example of a grating coupler that can be manufactured by use of a silicon-on-insulator (SOI) wafer; and the grating coupler operates to receive an optical signal via an optical waveguide, and convert an optical axis as a result of diffraction. The cross-section structure shown as an example in FIG. 1 comprises a BOX (Buried Oxide; buried oxide film) layer 102 comprising silicon dioxide (SiO2), a core layer 114 comprising silicon (Si) which has a refractive index higher than that of the BOX layer 102, and an upper cladding (overclad) layer 116 comprising silicon dioxide which is the same as that in the BOX layer 102 or a material having a refractive index similar to that of the silicon dioxide in the BOX layer 102, wherein these layers are stacked in this order on a substrate 101; and a diffraction grating is formed on the core layer 114. In this manner, i.e., by simply processing the core 114 to form a diffraction grating thereon for example, the grating coupler is made to be able to perform optical-path conversion.
In the following description, an outline of a principle of operation and so on of a grating coupler for outputting guided light to the outside of an optical waveguide will be explained. Note that, although the case wherein guided light is outputted to the outside of an optical waveguide by a grating coupler will be explained in the following description, it is also possible to input external light into an optical waveguide by performing the operation in a reverse manner.
On the core layer 114 of the grating coupler 100 shown in FIG. 1, a diffraction grating is formed, wherein the diffraction grating has concavities and convexities in the direction of thickness of the waveguide (x direction) and the concavities and convexities are arranged periodically in the direction of propagation of guided light (z direction). In the case that a diffraction grating such as that described above is formed, a part of guided light is diffracted and emitted; wherein, the guided light and the emitted light satisfy a requirement for phase matching in the propagation direction (z direction). Specifically, in the case that the propagation constant of the guided light is βo, and the propagation constant of the emitted light in the z direction is βq, the requirement for phase matching is as follows:βq=βo+qK  (Formula 1)In this regard, note that q is a value corresponding to an order number (0, ±1, ±2, . . . ) of the emitted light, when it is defined that K=2π/Λ, wherein Λ is the period of the diffraction grating.
In this case, an emission angle θ of the emitted light with respect to a normal line of the diffraction grating can be obtained by use of the formulanc sin θ=N+qλ/Λ  (Formula 2)wherein λ denotes a wavelength of light in a vacuum, N denotes an effective refractive index of the waveguide, nc denotes a refractive index of the upper cladding layer, and Λ denotes a period of the diffraction grating.
In general, since the effective refractive index N of the waveguide takes a value between a value of a refractive index of a core and a value of a refractive index of a cladding, their relationship is represented as N>nc; and the light is emitted only when the order number q satisfies q≤−1. Further, since efficiency of diffraction becomes higher when the order of diffraction of diffracted light is smaller, the ratio of power distribution to the emitted light is maximized when emitted light satisfying q=−1 is used. Note that the above −1st order emitted light includes two types of emitted light, i.e., upwardly emitted light Pup and downwardly emitted light Pdown that travels toward the substrate 101 side via the BOX layer 102. Also, at the same time, light Pref returning in the direction toward the waveguide and light Ptrans transmitting through the core layer exist. There is the following relationship, in the case that light inputted to the grating coupler is denoted by Pin:Pin=Pup+Pdown+Ptrans+Pref  (Formula 3)
The emission angle θ of the upwardly emitted light Pup can be freely designed by determining the period Λ, the width w, and the depth d of the grating and the thickness D of the optical waveguide. For example, in the case that the material of the core layer 114 is silicon and the material of the BOX layer 102 is silicon dioxide (SiO2), ranges of values of the period A, the width w, the depth d, and the optical waveguide thickness D, for light having a wavelength in a range between 1.3 μm to 1.6 μm, are as follows:                Λ: 530-550 nm        FF (=1−w/Λ): 0.3-0.6        d: 60-80 nm        D: 180-220 nm        
In the case that the grating coupler is used as an optical coupler for outputting light, one of performance indexes is a ratio of the upwardly emitted light Pup to the input light Pin inputted to the grating coupler. The above index is named upward emission efficiency ηup, and the upward emission efficiency ηup is given as follows:ηup=Pup/Pin  (Formula 4)It is natural in an optical coupler that it is preferable to make the upward emission efficiency ηup to have a value close to 1; so that, as would be understandable from Formula 3, it is preferable to set Pup to be large, and set Pdown, Ptrans, and Pref as small as possible.
Non-Patent Literature 1 discloses, with respect to a grating coupler formed on a SOI wafer such as that shown in FIG. 1, a basic policy of designing for increasing Pup and reducing Pdown. One designing policy thereof is to set the thickness of the core layer 114 to have a value that is an integer multiple of ½ of a wavelength of light in a material forming the core layer 114, and another designing policy thereof is to set the thickness of the BOX layer 102 to have a value that is an odd-number multiple of ¼ of a wavelength of light in a material forming the BOX layer. By setting the thickness of the core layer 114 to have a value that is ½ of a wavelength of light, the phase of light that is directly and upwardly scattered by the concavities and convexities in the grating on the surface of the core layer and the phase of the light that is downwardly scattered and then returned after reflected at the back surface of the core layer 114 are aligned with each other (in-phase). As a result, the upwardly scattered light and the returned light constructively interfere with each other without cancelling one another, and the constructively interfered light is emitted upwardly; thus, Pup increases. On the other hand, by setting the thickness of the BOX layer 102 to have a value that is an odd-number multiple of ¼ of a wavelength of light, the light that passes through the back surface of the core layer 114 and enters the BOX layer and, thereafter, is reflected by the back surface of the BOX layer 102 and returned and the light that is reflected by the back surface of the core layer 114 and directly oriented upward constructively interfere with each other without cancelling one another. As a result, many pieces of light in the downwardly scattered light, that was once scattered downwardly form the surface of the core layer 114, return upwardly, so that the ratio of Pdown relatively decreases. As explained above, Pup increases and Pdown decreases by appropriately designing the thickness of a SOI layer and the thickness of a BOX layer, so that the upward emission efficiency ηup can be made to be large up to a certain magnitude.
However, in the case that reflection of the back surface of the SOI layer and/or the back surface of the BOX layer is simply maximized, it is not possible to sufficiently reduce the amount of the downwardly emitted light, so that the degree of increasing of the upward emission efficiency ηup is limited. It is possible to reduce the amount of the downwardly emitted light by providing the back surface of the grating with a metal reflection film or a reflection film comprising a multilayer film; however, since the manufacturing process for such a construction will be complex, this idea is impractical. Thus, a grating structure, which is based on a different principle of operation, for further increasing the upward emission efficiency ηup was created.
In the construction of the grating, it is attempted to increase the upward emission efficiency ηup by making scattered light, that is scattered by concavities and convexities on a grating, to be efficiently scattered only in the upward direction, originally. For example, a structure of a grating by which light is efficiently scattered in an upward direction is disclosed in Non-Patent Literature 2, wherein a concavity and convexity part of the grating is formed to have an asymmetrical saw blade shape, for making successful interference of scattered light to be occurred within an SOI layer so as to make phases of pieces of upwardly scattered light only are to be aligned to each other.
Further, Non-Patent Literature 3 discloses a structure, wherein a grating part is made to be thick by stacking polysilicon, amorphous silicon, or the like, and, thereafter, deep grooves of a grating are made thereon. FIG. 2 is a schematic figure of a cross section of a grating disclosed in Non-Patent Literature 3, for explaining an operation of the grating. As shown in FIG. 2, in the grating structure, scattered light is mainly generated at corners on the bottoms of the grooves in the grating. That is, inputted light is scattered at the corners on the bottoms of the concavity and convexity part of the grating; and pieces of downwardly scattered light 230 and 231 are generated, wherein one of the pieces of the downwardly scattered light travels through silicon in the SOI layer in a manner similar to that of another of the pieces of the downwardly scattered light. The period of the grating is selected in such a manner that the phases of the pieces of light are different by an odd-number multiple of n from each other, i.e., have phases opposite to each other, at the time that the pieces of light are generated. Thus, the pieces of downwardly scattered light destructively interfere with each other, and cancel out with each other. On the other hand, among pieces of the upwardly scattered light, even if the phase of light 232, that is scattered upwardly through a concavity, in the scattered light is shifted by π from the phase of light 233, that is scattered upwardly through a convexity, in the scattered light when they are generated, the phase shift disappears while the pieces of light propagate through a path having a different refractive index; and the phases of the pieces of light become the same when they arrive at the top part of the grating. Therefore, the light 232 and the light 233 constructively interfere with each other, and are efficiently emitted in the upward direction. For making reversed-phase and/or in-phase interference to be occurred, the thickness of the SOI layer and/or the depth of the groove are/is adjusted. Silicon layer 242 stacked on the grating part is referred to as an “overlay” to the SOI layer. In addition to silicon, a dielectric material having a high refractive index, such as a silicon nitride film (Si3N4 film), may be used as a material of the overlay, according to an available processing technique.
As explained above, the prior-art structures of the gratings disclosed in Non-Patent Literature 2 and Non-Patent Literature 3 are superior in the effect for increasing upwardly emitted light Pup and reducing downwardly emitted light Pdown, when the grating structures only are viewed. However, in actuality, an expected degree of upward emission efficiency ηup may not be obtained, due to a variety of restrictions. For example, regarding the structure of the grating disclosed in Non-Patent Literature 2, it is difficult to form the structure by use of a conventional dry etching device, since the structure has an asymmetrical saw blade shape. Thus, an ideal saw blade shape may not be formed and, accordingly, actual efficiency of upward emission may be lowered.
The above problem does not occur in the grating structure disclosed in Non-Patent Literature 3; however, another problem relating to adoption of deep grooves occurs therein. Specifically, as shown in FIG. 2, a gap 240 of a deep groove suddenly appears in front of light 250 when the light 250 enters the grating part from the side of the waveguide of the core layer 214; thus, there is tendency that the input light is reflected at the position from where the grating starts, and a large amount of reflected light Pref is thereby generated. As a result, upward emission efficiency ηup may be lowered, and/or noise may be generated in an optical circuit connected to an upstream side of the grating. In a prior-art technique, for solving the above problem, an apodizing process is applied to make a groove positioned at the beginning part of the grating, which is connected to a waveguide, to be narrow, shallow, or the like. However, it is necessary to finely process the groove for obtaining sufficient effect in terms of reduction of reflection; so that it is difficult to manufacture, with a high yield, gratings which respectively have small input reflection losses.